Stack Overflow tags - How do they display info on mouse over?

Question

At the event of mouse over each Stack Overflow tag displays an information box beneath itself. As long as the mouse is over the tag or over the info box, it continues to be visi

Answer 1

One of the many jQuery tooltip plugins:

http://jquery.bassistance.de/tooltip/demo/

Also, I think SO uses AJAX to load the information inside the tooltip, but you don't have to do that.